Two-way radios are typically designed with a robust amount of gain in their front end amplifier stages. As is well known in the art, one drawback of providing too much gain occurs when multiple communications systems are operated in close proximity to one another. The radio frequency (RF) gain stages of the radio receiver can both provide gain but can also act to enhance interfering signals. Generally, the cause of this type of problem stems from intermodulation distortion (IMD) and adjacent channel interference, which degrade radio performance in the form of poor radio reception. Intermodulation distortion interference occurs from adjacent channel interference that mixes with other RF signals to produce an unwanted RF signal on or near the receiver filter pass-band. This type of interference is becoming more common every day as radio spectrum becomes more crowded with differing types of users, RF signal power levels, and modulation schemes that all attempt to fit into a finite space.
In the past, one common way to help the receiver reduce this type of interference has been through the use of an attenuator device. The attenuator is a circuit the may be inserted between the antenna of the radio receiver and the RF amplifier circuitry in order to reduce the amount of RF energy reaching the receiver. As a general rule since the intermodulation product is a third order non-linear expression, every one decibel (dB) of attenuation that is switched in circuit provides a three dB reduction in the amount of intermodulation interference in the radio receiver.
One method currently used to control an attenuator for controlling this type of interference is through the use of a radio signal strength indication (RSSI) measurement. In this method, the signal strength is typically measured at the antenna or at some point in the RF amplifier chain in order to determine the absolute signal strength of the input radio signal. The signal strength may also be mathematically computed in a digital signal processor (DSP). If the radio signal reaches some predetermined threshold, then an attenuator may be switched into the receiving circuit in order to reduce the amount of interference that might be caused by this high signal condition. In some schemes, attenuation may even be switched in a stepped fashion where a greater amount of attenuation is used depending on the level of the RF input signal. Other schemes may use the RSSI measurement in conjunction with a carrier squelch (CSQ) detector circuit or software. However, CSQ is solely dependent on the RSSI measurement and not some “known” information.
The problem associated with these types of arrangements is that the receiver cannot determine if the received RF signal is an actual signal to be received or if the high RSSI level is merely interference or a combination of both. In general, when interference is present, the RSSI level is the sum of both the desired signal power and the interference power. Since both an on-channel signal and off-channel interference will provide a high RSSI level, the attenuator will continually be left in-line. This severely reduces the sensitivity of the receiver in situations where low RSSI levels need to be received. Communications are often missed since the receiver does not receive these lesser signals in view of the attenuator that is always in circuit. Still further methods have been developed and used in connection with the RSSI measurement such as measuring the incoming bit error rate (BER) against the signal strength to decide if the incoming signal is an actual signal or an interfering one. In many two way radio communications systems there is no digital bit information available to compute the BER. However, many two way radio communications systems have some form of squelch detect circuitry or software that utilizes “known” information for the purpose of unsquelching or unmuting the radio speaker. This information may be in the form of synchronization data as well as sub-audible tones such as a sinusoidal tone or a digital signal.
Thus, there is a need to provide a method for a radio receiver that requires the use of this “known” incoming data, for selectively switching an attenuator in connection with an RF input signal. This would work to ultimately optimize the performance of the radio receiver by enabling the user to accurately receive various signal levels of incoming information while still enabling an attenuator circuit in situations where adjacent channel interference or high on-channel signal levels are present.